In cardiac control devices, there is a fundamental advantage in performing a hemodynamic control method based on the measurement of cardiac output. This is illustrated by the three-phase relationship between cardiac output and pacing rate shown in a report by J. L. Wessale et al., entitled "Cardiac Output Versus Pacing Rate At Rest And With Exercise In Dogs With AV Block", PACE, Vol. 11, page 575 (1988). At low pacing rates (first phase), the cardiac output increases proportional to pacing rate. At some point (second phase), further increases in pacing rate cause the cardiac output to rise only slightly, if at all. At still higher pacing rates (third phase), further increases will cause the cardiac output to diminish. The width of the second phase is considered an indication of the pumping capacity of the ventricles and the health of the heart. Rate-responsive pacemakers cannot determine the phase of the cardiac output/pacing rate relationship for a given pacing rate without measuring cardiac output.
The most accurate method of measuring cardiac output is to directly measure the flow of blood from the heart using Doppler ultrasound techniques in which the ultrasound transducer interrogates the flow within the aorta near its attachment to the left ventricle. The best method for examining aortic blood flow, within the constraints of standard pacing or electrophysiological test procedures, entails measuring blood flow from a transducer mounted on a catheter within the right atrium or right ventricle of the heart.
The standard ultrasonic catheter comprises a catheter, ultrasound crystals mounted on the distal portion of the catheter, ultrasound processing electronics attached to the proximal end of the catheter, and a conductor for electrically connecting the crystals with the electronics. Millar, in U.S. Pat. No. 4,771,788, entitled "Doppler Tip Wire Guide", issued Sep. 20, 1988, discloses one example of an ultrasound catheter in which the connection between the crystals and electronics is a helically wound spring coil. This patent also discusses a second embodiment in which the catheter includes an elongate support wire and a pair of electrical leads encapsulated within an insulator sheath.
Unfortunately, transmit and receive signals on the electrical leads mutually interfere to create noise which is detected by the ultrasound processing electronics. One method of electrically separating the transmit and receive signals is to communicate them using coaxial cables. Johnston, in U.S. Pat. No. 4,802,490, entitled "Catheter for Performing Volumetric Flow Rate Determination in Intravascular Conduits", issued Feb. 7, 1989, discloses a catheter which communicates ultrasonic signals over cables.
There are disadvantages to a catheter which communicates signals to and from ultrasound crystals using coaxial cables. A coaxial cable is subject to large power losses. Also, a catheter which is chronically implanted within the heart cannot withstand the large number of flexures occurring over time.